The present invention relates to a needle roller bearing wherein a cage and a plurality of needle rollers are assembled. The needle roller bearing of the invention is suitable for use as a support bearing for a shaft (main shaft or counter shaft) or a gear (driven speed change gear) in a transmission used in automobiles, construction machines, agricultural machines and the like, a support bearing for a connecting rod in bicycle engines, or a support bearing for a planet gear in sun-and-planet gear devices.
In the transmission of an automobile or the like, the support section of the driven speed change gear has often employed therein a needle roller bearing with no bearing ring, wherein a cage and a plurality of needle rollers are assembled. This is for the purpose of reducing the size and weight of the transmission by making use of the advantages of this type of needle roller bearings that they are light in weight, small in sectional height and high in load capacity.
FIG. 8 shows a conventional needle roller bearing which supports the driven speed change gear of a transmission. This needle roller bearing comprises a cage 11 having a plurality of elongated window-shaped pockets 11a formed therein at predetermined circumferential intervals, and a plurality of needle rollers 12 received in the pockets 11a.
The cage 11 is an annular body having annular portions 11b on axially opposite sides of the pockets a, and a pillar portion 11c circumferentially continuous with said annular portions 11b on opposite sides. The axially middle portion 11c1 of the pillar portion 11c is dented so as to be smaller in diameter than the axially opposite portions 11c2, and the axially middle portion 11c1 and the axially opposite portions 11c2 are connected to each other through inclined portions 11c3. Further, the annular portions 11b are in the form of flanges projecting radially inward from the axially opposite portions 11c2 of the pillar portion 11c. As a result, the longitudinal section of the cage 11 including the pillar portion 11c is substantially M-shaped as a whole.
As shown enlarged in FIG. 8(b), the four corners A' of each pocket 11a of the cage 11 are each formed with a corner round 11a1 to relax stress concentration. The size (radius of curvature) of the corner round 11a1 is set at a smaller value than the chamfering size of a chamfer 12a on the end surface of the needle roller 12 so as to avoid interference with the needle roller 12.
A needle roller bearing incorporated in a transmission is lubricated with lubricating oil fed by the splash system or forced circulation system. If the lubricating oil has a large amount of foreign matter, such as abrasion powder from the material of gears, mixed therein, such foreign matter accumulates in the bearing, a fact which sometimes forms the main cause of lowering the bearing life. The reason is that the accumulation of foreign matter causes flaking which starts from indentations formed on the contact surface of the bearing by the foreign matter cutting thereinto or peeling due to the hindrance to the formation of lubricating oil films. Particularly, since the bearing which supports a driven speed change gear is operated under special load conditions (light load during idling with respect to the main shaft, and heavy load during rotating with the main shaft), the contact surface tends to be damaged owing to the accumulation of foreign matter or the hindrance to circulation of lubricating oil).
The problem of the bearing life being decreased owing to said accumulation of foreign matter or said hindrance to circulation of lubricating oil likewise occurs, for example, in a needle roller bearing supporting the connecting rod of an engine or a needle roller bearing incorporated in a sun-and-planet gear device. Particularly, a needle roller bearing supporting the large-diameter boss of a connecting rod is structurally subjected to large centrifugal force and inertial force, so that oil film exhaustion tends to occur in the slide surface of the cage and the like. Therefore, it is an important technical problem to enhance the discharge of accumulated foreign matter and the circulation of lubricating oil.